7075-T6 Failure 2

[sreid]

I am using some machined from solid aluminum flex shaft couplings made from 7075-T6 aluminum. Sometimes when the allen head clamp screws are over tightend, the coupling breaks at the screw head. This does not surprise me in that the screw shoulder is pulling right on the stress riser at the bottom of the screw head pocket and that's right where it snaps. The break shows no yield before breaking; the break surface looks like brittle fracture like a piece of chalk would look like if you snapped it. Is this typical for 7075-T6?

 

[unclesyd]

Could you come back with the enviroment the part sees.
What size is cross section that is failing?

It is not unusual not to see evidence of ductility in an aluminum fracture

 

[metman]

Sounds like a geometry problem in two ways.

1) Can you use a bullnose end mill to create the counter-bore (pocket) to minimize the stress riser?

2) If you really must stick with Aluminum for lightness, then maybe you can thicken the section underneath the bolt head to reduce nominal stress.

 

[dwightdixon]

If you want any elongation maybe T-6 is too hard for you. Maybe T-4 would be better but is considerably lower yield strength. T-6 is quite notch sensitive so maybe you could get away with proper radiusing the part. You really need to have the mechanics use torque wrenches, otherwise they will always twist them till they break.

 

[CoryPad]

You may need to change the fastener. If your allen screw is a socket head cap screw, then it has little bearing area, which means the Al part could be overstressed.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sreid]

Details of the coupling can be found below. It's an R12 clamp end 5/8" bore.

http://renbrandt.com/onep_details.html

My question is for information only, we are switching to a different coupling (primarilly for a higher torque rating, Renbrandt makes high quality couplings).

But it's the appearance of the fracture that got my attention. It looks like what I would expect to see in an aluminum casting.

 

[unclesyd]

As stated above the material has low ductility and is notch sensitive along with being somewhat prone to SCC under tension in short sections in the traverse direction to the grain structure (AlCOA).

Here is similar type coupling that we used with very good success that you may want to look at. The only time they broke was when we overwound them by various means.

http://www.heli-cal.com/

 

[metman]

sreid,
You said "...we are switching to a different coupling (primarilly for a higher torque rating..."

Does this mean that it was a geometry problem -- just not big enough or is it different material like alloy steel or...?

 

[swall]

To add to unclesyd's comments about SCC--it is widely recognized that 7075-T6 is extremely susceptible to SCC in the short transverse direction. 7075-T73 is the perfered alternitive for environments where SCC might be a concern.